In injectors operated by means of a servo valve, the nozzle needle of the injector is not directly connected to a piezo actuator with the expansion and contraction of said piezo actuator generating the opening and closing movement of the nozzle needle. Instead, a servo valve is located between the nozzle needle and the piezo actuator, the valve body of said servo valve connected to the piezo actuator and said servo valve lifted away from a valve seat due to the expansion of the actuator. This takes place against the force of a valve spring which presses the valve body against the valve seat. Lifting the valve body of the servo valve away from the valve seat results in fuel flow which leads to operation of the associated nozzle needle.
In injectors of this kind, both the profile of the servo valve movement and that of the nozzle needle movement must be precisely regulated in order to achieve injection of a precisely metered quantity of fuel. This is carried out, in some examples, such that the nozzle needle closing time is determined and regulated. To this end, a voltage must be applied to the piezo actuator after the servo valve is closed, and therefore the actuator continues to remain in force contact with the servo valve. However, this assumes that the idle travel of the piezo actuator is known. Therefore, the idle travel is adapted at the same time (by means of the pressure drop or by means of the change in frequency/amplitude of the piezo actuator during the activation). However, this known adaptation method is relatively slow, and the accuracy and robustness are dependent on operating conditions.